In the production of various products of sheet metal by pressing, use is often made of sheet metal blanks which are piled into stacks in which the individual blanks lie directly on top of one another. The sheet metal blanks often carry an oil film, which entails that they show a manifest tendency to adhere to one another. On removal of the sheet metal blanks for further processing in a press or press line, use is made of different forms of gripping devices which, for example, may consist of suction cups which are applied to the uppermost sheet in the stack and which lift the sheet from the stack and move the sheet into the tool in the press.
If the sheets consist of magnetic material, the adhesive action of the oil film can be cancelled out by magnetic separators which prevent the suction cups from taking two or more sheets at the same time, which would cause immediate operational stoppage in the pressing operation.
If the sheets consist of non-magnetic material, for example aluminium, the technique employing magnetic separation will not function, for which reason adhesion between the sheet blanks implies almost constant operational stoppage.
In other technical fields of application in which separation of members or objects disposed in a stack has been brought into consideration, use has been made of screw-shaped separator devices which, with their threads, grip in between adjacent objects and lift out the uppermost object from the stack. In such instance, the axes of rotation of the screws are parallel with the edge of the stack, i.e. also parallel with a normal to the plane of that object or member which is to be separated.
This technique, known from the packaging industry, functions well only in such contexts where those portions of the objects which are grasped by the screws are spaced apart from one another such that the screw thread can pass into these gaps without damaging the objects. On the other hand, it is impossible to apply this technique successfully in such cases in which the objects or members form a compact stack which, thus displays no spacings between those portions of the objects which serve for engagement for the screws.
Nor can the technique known from the packaging industry be employed in such cases in which those objects which are to be separated do not lie in well-arranged stacks, such that individual objects may project out laterally beyond the rest of the stack. Precisely such poor precision in the stacks, i.e. individual sheets or groups of sheets projecting laterally out from the stack is difficult to avoid in the feeding of sheets to, for example, a press.
Despite the above-outlined problems in compact stacks and employing screws whose axes of rotation are parallel with the edge of the stack, Russian patent specification 870 322 discloses a solution which is based on this principle. In the design and construction according to this Russian patent publication, use is made of a screw whose thread has a sharp knife edge which projects out along a radius and which is intended to cut in between two adjacent sheets. Naturally, extremely strict requirements on precision are placed here if such an apparatus is to be capable of separating from one another metal sheets of the order of thickness of between 0,5 and 1,0 mm. Even one or a couple of tenths of a millimetre of incorrect placement of the cutting edge in relation to the stack will result in the sheets being damaged and deformed--with an imminent risk of operational stoppage. There is a risk of exactly the same outcome if the sheets in the stack suffer from edge damage resulting, for example, from impact. How great these precision problems can be is most readily apparent from the fact that the sheets under consideration here are a square metre or more in size and that the stack may weigh several tonnes.